I am working on a feedback control application for a DC DC converter. I need to make a small circuit, which can sense 2 voltages (both in the range 50 - 74 V)
and generate a PWM signal whose duty cycle is proportional to the voltage difference.

The challenge is that I need to make this completely analog, i.e. I cannot use any ADCs or micro controllers.

I am just looking for ideas in terms of block diagrams and I am ready to do the menial op amp calculations myself.